Oil for any public vehicle, including the Mercedes E350, is available from most automotive parts stores, such as Advance Auto Parts. As long as the weight and type match the vehicle's manufacturer specifications, the oil will not harm the engine.
There are several types of oil brands on the market, all of which offer the synthetic 5w30 or 5w40 a Mercedes E350 needs. However, each oil brand makes use of a specific additive package, which helps lubricate and clean the engine's internal components. In general, the more expensive the additives used for an oil mixture, the higher the price per quart.